Oscillator clock circuits that utilize a crystal oscillator reference are widely used in electronic devices. One application of crystal oscillator-referenced clock circuits is in metrology circuits, such as in electricity meters.
The primary function of an electricity meter is to accurately measure energy consumption. Oscillator circuits are used in electricity meters to, among other things, provide clock signals for digital processing elements, and to provide real-time clock information for enhanced energy consumption metering modes. An example of an enhanced metering mode that requires real-time clock information is time-of-use metering. Time-of-use metering involves measuring electricity consumption during discrete time periods during a day, week, month and/or year and applying specific rates based on the time period. For example, energy used between 3:00 pm and 4:00 pm in the summer months may be charged at a higher rate than electricity used between 2:00 am and 3:00 am. Accordingly, the real-time clock allows application of the appropriate rate at the time that the energy consumption is measured. Electricity meters have other features, known in the art that also employ a real-time clock.
Features of an electricity meter that rely on a real-time clock usually require a high-accuracy clock. An inaccurate clock can lead to, for example, misapplication of time-specific cost rates, thereby causing billing errors. Clock accuracy is typically ensured by a timing reference that accurately tracks time. In many cases, the timing reference in an electricity meter may be derived from the 60 Hz signal of the mains AC power. One drawback of using mains AC power as a clock reference is the loss of the reference in the event of a power interruption. In particular, if there is a power interruption, the clock loses its AC reference and can fail or drift. Another source of an accurate timing reference is a crystal oscillator circuit. A crystal oscillator circuit having high accuracy may be used as a frequency reference which can provide continued accuracy during power interruption, so long as the meter circuits have power from a back up source such as a battery. Accordingly, many meters employ crystal oscillator circuits as a stable frequency reference.
In addition to metering features that employ a real-time clock, meters have increasingly incorporated automated meter reading (“AMR”) technology which includes a communication circuit that allows meter information to be gathered remotely. One of the main AMR technologies incorporates a radio that is connected to the meter. Various implementations of AMR radios employ different power levels and operating frequencies.
Unfortunately, the implementation of AMR radios can lead to the introduction of electromagnetic interference in the meter circuits. Typically, the radio and its antenna are mounted inside the meter in very close proximity to the metrology electronics. Because of the close proximity, the operation of radios can undesirably affect meter performance. In particular, a widely used radios are the 900 MHz unrestricted band (902 MHz to 928 MHz), with Frequency Hopping Spread Spectrum (FHSS). Accordingly, interference often occurs in the form of radiated signals having a frequency in the vicinity of 1 GHz. The problems of RF signal interference has been exacerbated by the more recent use of higher power radios. Specifically, although radios were first introduced at low power ranges (100 mW), it is not uncommon to employ AMR radios at levels of 250 mW, 500 mW, and 1 Watt.
A specific area of concern is the crystal oscillator reference frequency circuit. Electricity meters employ crystal oscillators to provide a stable time base that is important for many purposes, at least some of which would be equally applicable to microprocessor based gas and water meters. Induced RF noise on the crystal oscillator circuit can result in disruptive errors in the clock circuit operation.
More specifically, FIG. 4 discloses an exemplary Pierce oscillator circuit employing a crystal resonator, which may be used in a typical electricity meter. The operating principle of the Pierce oscillator circuit is to generate a stable and accurate frequency for meter operation. In general, the Pierce oscillator (or crystal oscillator) circuit generates a stable and accurate frequency that is used in conjunction with a Phase Locked Loop circuit, not shown, to generate clocking signals for the microprocessor within the processor chip package 12. The only external components for the oscillator circuit are a crystal resonator and two load capacitors C1, C2. It will also be appreciated that the chip package 12 typically incorporates electrostatic discharge Zener diodes 16 for ESD protection.
RF power from a radio installed in the meter housing or external can be picked up by the ground plane due to the relative magnitude of the signals, as well as the size of the plane and traces on the board. The power of the induced RF signal is proportional to the plane and trace sizes (acting as antennas), the transmitted power (higher power, close proximity of the transmitter, or both), and the frequency. At higher frequencies, smaller traces become more effective antennas, per the formula: λ=C/f, where λ is the wavelength, C is the speed of light 3×10 ^8 meters per second, and f is the operating frequency.
The ESD diode 16 is typically built into processing chips at input and output pins, and is referenced to the ground, supply voltage rail, or both. Such ESD diodes can rectify the induced RF signals and convert them to DC voltages, which can in turn alter the bias voltage at the respective input and output pins, or even causing current to flow. The DC voltage created by the rectified RF signals can cause errors in the internal clock circuits due to the topology of the clock circuits. In some cases, the errors can cause the clock circuit to fail to generate any stable frequency, resulting in a completely inoperative clock. As is well known, an inoperative clock can result in overall device malfunction.
Even with minor disturbances, the induced RF noise, depending on its severity can alter the amplitude as well as the phase of the oscillator which in turn will influence the microprocessor operation in undesirable manner.
In particular, the RF noise can couple to the oscillator circuit either through the crystal oscillator load capacitors (22, and 27 pF) as they provide relatively low impedance path:Xc=1/[(2×μ×f(915 MHz)×C(22 pF)]=8 Ωor directly to the traces between the external crystal oscillator circuit and the microprocessor.
In the art, it has been suggested to connect a ferrite bead between ground and a node, and connecting the capacitors to each other at that node. This is disclosed in Teridian EMC design application note; AN_6552_041_v2-2 EMC Design.
However, this topology has been found to provide insufficient noise suppression in metering applications.